In recent years, tire manufacturers have been making considerable efforts to develop novel solutions to a problem dating from the very first use of tired wheels of the inflatable type, namely the problem of how to allow the vehicle to continue to travel despite a considerable or complete loss of pressure of one or more tires. For decades, the spare tire was considered to be the only and universal solution. Then, more recently, the substantial advantages associated with its possible omission have become apparent. The concept of “extended mobility” was developed. The associated techniques allow the same tire to run, depending on certain limits to be respected, after a puncture or a drop in pressure. This makes it possible for example to drive to a breakdown point without having to stop, often in hazardous circumstances, to fit the spare tire.
Self-sealing compositions that allow such an objective to be achieved, and which by definition are capable of automatically ensuring, that is to say without external intervention, that a tire is sealed in the event of a perforation of the latter by a foreign body, such as a nail, are particularly difficult to develop.
To be useable, a self-sealing layer must satisfy many conditions of a physical and chemical nature. In particular, it must be effective over a very wide range of operating temperatures, and be so throughout the lifetime of the tires. It must be capable of closing off the hole when the perforating object remains in place and, when the latter is expelled, said self-sealing layer must be able to fill the hole and seal the tire.
Many solutions have admittedly been devised but they have not been able truly to be developed hitherto for vehicle tires, in particular because of difficulties in manufacturing these self-sealing compositions, and therefore because of their final cost of manufacture.
In particular, effective self-sealing compositions, based on natural rubber and on hydrocarbon resins as tackifier, have been described in the U.S. Pat. Nos. 4,913,209, 5,085,942 and U.S. Pat. No. 5,295,525. These compositions are characterized by the combined presence of a high content of hydrocarbon resin, always greater than 100 phr, and a large amount of elastomer in the liquid state, generally in the form of depolymerized natural rubber (molecular weight typically between 1000 and 100 000).
Firstly, such a high resin content, apart from the fact that it may be prejudicial to hysteresis and consequently to the rolling resistance of tires, requires particularly long and difficult kneading of the elastomeric matrix.
The use of a large amount of liquid elastomer certainly improves the fluidity of the composition, but such a use is a source of other drawbacks, especially a risk of the self-sealing composition creeping when it is used at a relatively high temperature (typically above 60° C.), as frequently encountered when certain tires are used.
If the liquid elastomer content is reduced or even if it is completely eliminated, another major manufacturing problem may then arise: in the absence of a reinforcing filler such as carbon black (which is moreover undesirable, as is known, for this type of application), the composition is not cohesive. This lack of cohesion may be such that the tack of the composition, resulting from the high content of tackifier employed, is no longer compensated for and carries it away. It then follows that there is a risk of the composition undesirably sticking on the compounding tools, which is unacceptable under industrial operating conditions.